In December 2015, more than 190 countries—representing the source of more than 95 percent of global greenhouse gas emissions—met in France for the United Nations Climate Change Conference. Together, they crafted the Paris Agreement on Climate Change.

“It’s the first time essentially all the countries of the world have agreed to a common framework for addressing the problem of climate change,” says Andrew Light, University Professor of Philosophy and Public Policy and director of the Institute for Philosophy and Public Policy at George Mason University. He’s also a distinguished senior fellow in the climate program at the World Resources Institute.

He calls the agreement an “absolute breakthrough, both in international diplomacy and in climate policy. It’s frankly going to give us the best shot that we’ve ever had for seriously dealing with the problem.”

On November 3 at 5 p.m. in the Hawkins-Carlson Room at Rush Rhees Library, Light will deliver a public lecture—“The Pragmatic Achievement of the Paris Climate Agreement”—as part of the Humanities Center Lecture Series, focused this year on the theme of the environment. He’ll describe the potential impact of the agreement and explain how negotiators in Paris overcame the core problem of distributive justice—the fair allocation of resources—that made the process so difficult.

In 1992, the United Nations Framework Convention on Climate Change split the world’s countries into two groups: developed and developing nations. When that body created their first agreement to reduce emissions — the Kyoto Protocol in 1997 — developed countries had binding legal obligations to reduce emissions, but developing countries did not.

“And the atmosphere can’t tolerate that,” says Light—because among the developing countries were nations such as China and India, which are now among the world’s top-10 emitters.

So the new agreement has no bifurcated categories. Instead, it’s been built on a system of “broad differentiation”—each country is essentially its own category, with its own profile, particular needs, and self-determined plan to cut emissions. Says Light: “It’s a very different structure where everyone says what they’re going to do and then there are mechanisms to pressure them, to raise expectations that they’ll do more.

Light has two distinct careers: one as an academic philosopher, and another as a hands-on climate change policy negotiator. From 2013 to 2016, he served as senior adviser and India counselor to the U.S. special envoy on climate change, and as staff climate adviser in the Secretary of State’s Office of Policy Planning in the U.S. Department of State. In that job, he was on the senior strategy team for the U.N. climate negotiations, directed the U.S.-India bilateral climate change working group, and chaired the U.S. interagency working group on climate change for the Post-2015 UN development agenda.

In the policy world, Light relies on his expertise in international climate policy, not his work as an ethicist—which he says would leave him “pigeon-holed,” dealing with narrowly conceived ethical issues—but he finds his philosophical training naturally applicable to the issue of climate change.

“I think the entire problem is a moral dilemma. It’s the biggest problem of distributive justice I think humans have ever faced,” he says. “Because fundamentally, you’ve got an atmospheric commons, and it’s being polluted by everyone in the world.” Reducing emissions requires that countries transform their economies, even as they strive for economic growth, which historically has gone hand-in-hand with energy production and its byproduct, carbon pollution.

“And so the fundamental question is, who cuts their emissions, by how much, and on what schedule?” Light says.

Understanding other countries’ perspectives in addressing the problem is where he sees the tightest overlap between his philosophical views and his diplomatic work.

“A good chunk of my work at the State Department was trying to help people in the U.S. government understand why the Indians were taking the positions that they were taking in the run-up to the negotiations,” he says. Each participant in the negotiations had to be able simultaneously to contribute to climate change solutions and to return home to show the country’s people that the agreement is good for them.

“You had to meet [other negotiators] on their terms and they had to be able to take home the same [agreement] text that we were going to take home. They were going to frame it in a different way than we were going to frame it, and we needed to be OK with that going in. Not only OK with it—we needed to support the fact that they were going to do that.”

When he speaks on climate change, people often comment on Light’s optimism. But he says it’s not really optimism—it’s understanding the ways in which the world is working seriously to address climate change.

“One of the things that made Paris different was moral leadership on the part of people like [President] Obama and [Indian] Prime Minister Modi,” he says. “They came to recognize that they both think they have obligations to future generations, and that that should drive their approach to doing something on the issue.”